15: Molarity, pH Scale and Strengths of Acids and Alkalis

Question 1

What is molarity?

Answer 1

Molarity is the number of moles of solute dissolved in one cubic decimetre of solution.

Question 2

What is the colour change when litmus solution is added to an acid?

Answer 2

It turns from purple to red.

Question 3

What is the colour change when litmus solution is added to an alkali?

Answer 3

It turns from purple to blue.

Question 4

What is the colour change of blue litmus paper in an acid?

Answer 4

It turns from blue to red.

Question 5

What is the colour change of red litmus paper in an alkali?

Answer 5

It turns from red to blue.

Question 6

What is the colour of methyl orange indicator in dilute hydrochloric acid?

Answer 6

red

Question 7

What is the colour of phenolphthalein in dilute hydrochloric acid?

Answer 7

colourless

Question 8

What is the colour of methyl orange indicator in dilute sodium hydroxide solution?

Answer 8

yellow

Question 9

What is the colour of phenolphthalein in dilute sodium hydroxide solution?

Answer 9

pink

Question 10

What is the definition of pH?

Answer 10

pH = -log[H⁺(aq)]

Question 11

How is the concentration of hydrogen ions calculated with given pH?

Answer 11

[H⁺(aq)] = 10^-pH

Question 12

What is the range of the pH scale?

Answer 12

from 0 to 14

Question 13

Are pH values lower than 0 or higher than 14 possible?

Answer 13

Yes, they are possible for very concentrated acid or alkaline solutions.

Question 14

What is the pH range of an acidic solution?

Answer 14

pH < 7

Question 15

What is the pH range of an alkaline solution?

Answer 15

pH > 7

Question 16

What is the pH range of a neutral solution?

Answer 16

pH = 7

Question 17

When does [H⁺(aq)][OH^-(aq)] = 10^-14 hold?

Answer 17

At 25ºC

Question 18

Why does pure water have a pH of 7 at 25ºC?

Answer 18

At 25ºC, we have [H⁺(aq)][OH^-(aq)] = 10^-14. In addition, water dissociates into hydrogen ions and hydroxide ions in a 1:1 mole ratio. Therefore, [H⁺(aq)] = 10^-7, thus water has a pH of 7 at 25ºC.

Question 19

Is the pH of boiling pure water the same as that as pure water at 25ºC?

Answer 19

No, the pH of boiling pure water is lower, however, boiling pure water is still neutral.

Question 20

List 4 methods to measure the pH of a solution.

Answer 20

1. Using universal indicator solution.
2. Using pH paper.
3. Using a pH meter.
4. Using a pH sensor connected to a data-logger.

Question 21

What is the definition of strong acid?

Answer 21

A strong acid is an acid that dissociates almost completely in water to give hydrogen ions.

Question 22

What is the definition of weak acid?

Answer 22

A weak acid is an acid that dissociates partially in water to give hydrogen ions.

Question 23

Which type of arrow is used for the dissociation of hydrochloric acid in water?

Answer 23

Single arrow, since all the molecules dissociate into hydrogen ions and chloride ions.

Question 24

Which type of arrow is used for the dissociation of ethanoic acid in water?

Answer 24

Double arrow, since not all the molecules dissociate into hydrogen ions and ethanoate ions.

Question 25

List 3 methods to compare the strengths of acids.

Answer 25

1. Measuring the **pH value** of the acids in the same concentration. 2. Measuring the **electrical conductivity** of the acids in the same concentration. 3. By comparing the **rate of reaction** of the acids with metals in the same concentration.

Question 26

Why does a strong acid have better electrical conductivity than weak acids?

Answer 26

The electrical conductivity of a solution is proportional to the concentration of **mobile ions**. Strong acids **dissociate more** compared to weak acids, so the concentration of mobile ions is higher. Thus, they have better electrical conductivity.

Question 27

Why does a strong acid have a higher rate of reaction with magnesium than weak acids?

Answer 27

Strong acids have a **higher concentration** of hydrogen ions, thus they react **more rapidly** than weak acids with magnesium to form hydrogen gas.

Question 28

Magnesium ribbons is added to two test tubes of strong acid and weak acid of the same concentration. Do the two test tubes produce the same amount of hydrogen gas?

Answer 28

Yes. Although the dissociation of weak acids is incomplete, the hydrogen ions are being **consumed** in the traction with the magnesium, and the reaction is **pushed to completion**. Eventually, all hydrogen atoms in the weak acid will be released as hydrogen ions and reacted to make hydrogen gas.

Question 29

What is the definition of strong alkali?

Answer 29

A strong alkali is an alkali that **dissociates almost completely** in water to give hydroxide ions.

Question 30

What is the definition of weak alkali?

Answer 30

A weak alkali is an alkali that **dissociates partially** in water to give hydroxide ions.